Numb chin syndrome (NCS) was initially described in the 1830s by Charles Bell when he noted an absence of sensation in the left lower lip of a patient he was seeing for breast cancer. During her examination, he noted a palpable mass extending along the angle of her jaw, presumably into the mental foramen. The term "syndrome of the numb chin" did not arrive, however, until the 1960s when Calverley and Mohnac described five patients with chin numbness from various malignancies [1]. Though it is now understood that NCS may arise from a variety of conditions, it is the known potential association with malignancy that is most concerning.

Given the relatively small area involved, it is possible for both patients and clinicians to underemphasize this symptom. It is the goal of this topic to improve the understanding of the widespread differential of this potentially ominous sign. This topic will review the anatomy, clinical features, differential diagnosis, evaluation, and prognosis for NCS.

NERVE ANATOMY

The inferior alveolar nerve travels through the medial surface of the mandible though the mandibular canal, dividing into the incisive and mental branches just anterior to the first premolar tooth (figure 1 and figure 2). The incisive branch innervates the first premolar, canine, and incisor teeth along with the associated vestibular gingiva. The mental nerve passes through the mental foramen on the anterior mandible, typically below the second premolar tooth, before dividing into two or three branches that supply sensation to the skin of the chin, the mucous membrane of the lower lip, and the buccal gingivae of the mandibular teeth and premolars [2,3]. These branches typically stop at midline [4].

Moving proximally, the inferior alveolar nerve is a branch off the posterior portion of the mandibular nerve, which passes through the foramen ovale before connecting to the trigeminal ganglion in Meckel's cave. This sends afferent sensory trigeminal fibers into the lateral pons, coursing dorsomedially to the principal sensory and spinal trigeminal nuclei in the brainstem.

PATHOPHYSIOLOGY

Numb chin syndrome (NCS) can occur from a lesion anywhere along the course of the trigeminal nerve, including proximally within the Gasserian ganglion or even the nerve cell bodies within the pons [5]. Lesions can be broadly divided into peripheral lesions (involving the mandible or direct nerve infiltration, inflammation, or compression) and central lesions (involving the base of skull, leptomeninges, or brainstem). In the case of malignancy-associated NCS, neuropathy can be from direct infiltration of the nerve or mandible, such as from melanoma of the oral mucosa, or metastasis from a distant neoplasm.

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